gnome-shell: Add transition to window-close buttons

This was added upstream in GNOME 40 cycle [1]. Use faster than
upstream transition for Arc, based on panel button transitions.

[1] https://gitlab.gnome.org/GNOME/gnome-shell/-/commit/97b9cdc720d2b7699c708594cfda7ef8c0531ac6

Author: jnsh

common/gnome-shell/40/sass/_common.scss

@@ -1568,6 +1568,7 @@ $window_close_button_padding: 0;
   height: $window_close_button_size;
   width: $window_close_button_size;
   padding: $window_close_button_padding;
+  transition-duration: 100ms;
 
   & > StIcon { icon-size: 16px; }